Conventional precision casting techniques typically begin with the making of a replica, pattern or form of a workpiece to be made, around which molding material is packed to shape the casting cavity of a mold.
Cores are shapes of sand that are placed in the mold to provide castings with contours, cavities and passages. Cores are composed mainly of sand, but also contain one or more binder materials. Sand cores are made in core boxes. A core box, preferably formed from metal, is machined to include portions which define the main body of the core. The shape of the core conforms to the shape of the walls of the workpiece or fixture to be cast. The core is formed by blowing a sand/binder/air mix into the core box under pressure.
After the core is removed from the core box, the core proceeds to subsequent traditional steps of the casting process for producing the desired workpiece. For brevity, those steps will not be described here.
Conventionally, the core is formed from a fine grain sand mixture of approximately two-thirds silica sand and one-third chromite. The sand may be coated with a phenolic resin binder. Core sands are usually silica sands, but zircon, olivine, chromite, carbon and chamotte sands are used also. Coarser sands permit greater permeability and often are preferred for cores. In selecting a core sand, it must be considered that most of the sand from burned-out core centers enters the molding-sand system and may well be a significant portion of the system sand. Under these conditions, a sand must be selected that will be suitable for both molds and cores.
Core blowing is achieved by filling the core box cavity with sand that is suspended in a stream of air and introduced into the cavity at high velocity through blowing holes. The sand is retained by the cavity walls and the air is exhausted through vent holes in the core box. Core blowing is a high-speed operation. A core blowing machine, under proper conditions, will deliver the core sand mixture into a core box within a few seconds, depending on the size of the machine. More time is necessary before the core is removed from the box, depending on whether the binder system being used requires further hardening or curing of the core while it is in the box.
In a known apparatus for core blowing, there is an air inlet, and a chamber which directs air through a sand mixture to form a sand and air mixture which is propelled into the core box cavity. The air is then exhausted from the core box cavity via screened air vents. Conventionally, the location of such screens allows the sand to flow to a dead-end spot, while allowing air to escape. There are other conventional components which are not described here for brevity, including blow valves and exhaust ports.
Following the traditional approaches, the screened air vents often become occluded. This tends to affect the flow of material within the core box and ultimately manifests itself in an impaired product quality and excessive machine downtime during which clogged vents are cleaned. This cleaning is necessitated by the use of sand, resins, and high air pressure in the production of sand cores for cast metal parts. Resin and sand accumulate in the core box passageways and on certain surfaces such as the screens within the core box.
Since the solid core defines voids in the cast product, there is an intimate relationship between the quality of the core and the quality of the resulting product after casting. Accordingly, a need has arisen to engineer core boxes which are used to make the core to increasingly high levels of precision in order to ensure that a solid core is prepared with sufficiently defined and controlled surface characteristics which are ultimately imparted to the cast product.
Conventional foundry practices include the use of abrasive techniques for core box cleaning, wherein the medium is carbon dioxide, glass beads, sand, walnut shells, or sodium bicarbonate. Liquid core box cleaning techniques call for immersion in caustic solution, perhaps enhanced by heat and ultra-sound, or the use of steam under high pressure.
Existing approaches require that the core box be removed from the production machine for cleaning and inspection. Such steps are labor-intensive, time-consuming, and costly. In some cases, where hot caustic solutions are used for cleaning, the steps involved are ergonomically and environmentally hazardous.
In U.S. Pat. No. 4,048,709, a method is disclosed for producing cast metal parts with smooth surfaces. The method includes making a core box and pattern to closer tolerances than the finished part. U.S. Pat. No. 3,633,010 discloses a computer-aided laser-based measurement system in which the surface of a specimen to be evaluated is scanned incrementally by a laser beam in response to program signals generated by a computer. U.S. Pat. No. 4,620,353 discloses a system for electro-optical and robotic casting quality assurance.